Nassim Nicholas Taleb's book The Black Swan.
Taleb writes that the medievals thought all swans were white. Then in 1697 Willem de Vlamingh saw black swans in Australia and millennia of empirical evidence were disproved. Knowledge is fragile, life is uncertain. You can suddenly see a black swan. Or neutrinos can go faster than light! The trick is to be robust in the face of negative black swans and expose ourselves to as many positive black swans as possible. I find this strangely comforting.
Taleb's first black swan came when the Lebanese civil war started in 1975. I grew up with people whose black swan was the loss of their home, culture, community and language—they were anxious but also amazingly resilient to change. They taught me that when the unthinkable happens, you cope and hope. They made me an optimist.